Schüllerite, Ba2Na(Mn,Ca)(Fe3+,Mg,Fe2+)2Ti2(Si2O7)2(O,F)4, a new mineral species from the Eifel volcanic district, Germany

Abstract

A new heterophyllosilicate mineral schüllerite was found in the Löhley basalt quarry in the Eifel volcanic region, Germany, as a member of the late mineral assemblage comprising nepheline, leucite, augite, phlogopite, magnetite, titanite, fresnoite, barytolamprophyllite, fluorapatite, perovskite, and pyrochlore. Flattened brown crystals of schüllerite up to 0.5 × 1 × 2 mm in size and their aggregates occur in miarolic cavities of alkali basalt. The mineral is brittle, with a Mohs hardness 3–4 and perfect cleavage parallel to (001). D calc = 3.974 g/cm3. Its IR spectrum is individual and does not contain bands of OH−, CO 3 2− or H2O. Schüllerite is biaxial (−), α = 1.756(3), β = 1.773(4), γ = 1.780(4), 2V meas = 40(20)°. Dispersion is weak, r < ν. Pleochroism is medium X > Y > Z, brown to dark brown. Chemical composition (electron microprobe, mean of five-point analyses, Fe2+/Fe3+ ratio determined by the X-ray emission spectroscopic data, wt %): 3.55 Na2O, 0.55 K2O, 3.89 MgO, 2.62 CaO, 1.99 ArO, 28.09 BaO, 3.43 FeO, 8.89 Fe2O3, 1.33 Al2O3, 11.17 TiO2, 2.45 Nb2O5, 26.12 SiO2, 2.12 F, −0.89 -O=F2, 98.98 in total. The empirical formula is (Ba1.68Sr0.18K0.11Na1.05Ca0.43Mn0.47Mg0.88Fe 0.44 2+ Fe 1.02 3+ Ti1.28Nb0.17Al0.24)Σ7.95Si3.98O16.98F1.02. The crystal structure was refined on a single crystal. Schüllerite is triclinic, space group P1, unit cell parameters: a = 5.4027(1), b = 7.066(4), c = 10.2178(1)Å, α = 99.816(1), β = 99.624(1), γ = 90.084(1)°, V = 378.75(2) Å3, Z = 1. The strongest lines of the X-ray powder diffraction pattern [d, Å, (I, %)]: 9.96(29), 3.308(45), 3.203(29), 2.867(29), 2.791(100), 2.664(46), 2.609(36), 2.144(52). The mineral was named in honor of Willi Schüller (born 1953), an enthusiastic, prominent amateur mineral collector, and a specialist in the mineralogy of Eifel. Type specimens have been deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, registration no. 3995/1,2.